Can't say for sure since for starters I don't know what you're polling nor the capabilities of your SNMP graphing package. However, in the little snipet of CBWFQ stats you provided, it appears to be at least one other class, class-default, with another 1.5 Mbps shaper. In other word, don't know what other traffic your configuration might be permitting through the interface. My reply only addressed the information you had posted which didn't seem to account for the drop rate. You asked about a 27 Mbps shaped rate to a customer and showed individual class stats with such a class shaper. Now you mention interface bandwidths from SNMP interface data bandwidth usage graph. That's fine, but please try to post all relevant info and make clear what's your issue.

PS:

BTW, I don't have much expertise with SNMP packages or MIBs, i.e. if there's an issue there, doubt I'll be able to comment further. However, if you can limit yourself to posting the device's interface stats and showing more of the config, might be able to further resolve whether there is an issue with the shaper or whether it's just a lack of understanding with either its stats or how it functions.